Michael-type zwitterions, reactions

D. Further Reactions of the Initial Michael-Type Zwitterions... 

Figure 6.8 Proposed modes of action of hydrogen-bonding catalyst 16 Bidentate hydrogen bonding coordination of the zwitterion derived from Michael-type DABCO attack to methyl acrylate (1) and Zimmerman-Traxler transition state for the reaction of methyl acrylate with benzaldehyde (2).

Abstract Cascade reactions involving a transition metal-promoted step and a Michael-type addition process have emerged as a powerful tool to construct cyclic and polycyclic structures. In this review, recent advances in this field are presented. The first part is related to cycloaddition reactions based on zwitterionic jr-allylPd complexes. The second part deals with Michael initiated metal-catalyzed cyclofunctionalization reactions of unactivated C C jt-bonds. Parts three and four feature reactions where an initial Michael addition reaction is followed by either a coupling reaction or an electrophilic trapping. Part five is devoted to Michael terminated reactions. 

Reactions of carbene complexes with phosphorous compounds are topics of current research.164-167 (l-Alkynyl)carbene complexes (CO)5M= C(OEt) —C=CR 1 (M = Cr, W R = Ph, r-Bu, SiMe3) readily add tertiary phosphanes PPhR2 (R = Ph, Me) and form stable zwitterionic phosphonium allenide complexes (CO)5M —C(OEt)=C=C(R)—P+PhR2 145, which have been characterized by X-ray diffraction (Scheme 57).55 In previous studies on Michael-type additions of nitrogen and oxygen nucleophiles to (l-alkynyl)carbene complexes 1, zwitterionic allenide complexes of type 145 have been postulated as intermediates.21,23,503 62 112,168,169 Unambiguous structural evidence of the tendency to form such intermediates is given by isolation of a stable zwitterionic adduct, such as compound 145. Zwitterionic... 

Sulfonic acid functionalised ionic liquids may also be prepared via a zwitterionic intermediate from a Michael-type addition, as shown in Scheme 2.5.[84 85] In the first step 1-methylimidazole reacts with the sulfonic acid precusor 1,3-propane sultone to form a zwitterionic intermediate. Protonation with Bronsted acids affords ionic liquids with high purity that have proven to be highly efficient reaction media in, for example, esterification reactions.[86] Task-specific ionic liquids may also be prepared using semi-combinatorial methods.1871... 

The major adducts formed when phenanthridine is allowed to react with dimethyl acetylenedicarboxylate have been reformulated. - An initial Michael-type reaction gives the zwitterion (206) and succeeding reactions depend on the nature of the solvent. Adduct (207) is formed in anhydrous methanol by the addition of a proton and methoxide ion, while in benzene nucleophilic attack on the carbonyl group of a second ester molecule and subsequent cyclization provides 208. Alternatively, reaction at the triple bond of a second molecule of ester followed by ring closure of the new zwitterion gives 209. Other products related to 207 arise if the methanol contains water. 6-Methylphenanthridine with dimethyl acetylenedicarboxylate in benzene gives a mixture of the azepine (210) and tetramethyl 9a-methyl-9aA-dibenzo[a,c]quinolizine-6,7,8,9-tetracarboxylate. The... 

An ab initio smdy suggests that the formation of a Diels-Alder adduct [i.e. (23)] from reaction between 1-methylpyrrole and dimethyl acetylenedicarboxylate is favoured over the formation of a possible Michael-type addition product (24). Both pathways involve an initial electrophilic substitution at the 2-position of the pyrrole to form a zwitterionic intermediate. A review, in Chinese, concerns a number of electrophilic substitutions of pyridine. A kinetic study has been made of the reaction between 4,6-dinitrobenzofuroxan (25) and various anilines in acidic H2O-DMSO... 

One of the most important classes of Michael initiated ring closure processes in the construction of carbo- and heterocycles are stepwise cycloaddition reactions where a metal induces dipolar behavior in otherwise unreactive organic compounds to be reacted with activated olefins. In this area, Pd-assisted cycloaddition reactions which involve zwitterionic zr-allylPd complexes of type I (linear type), II, or III (Pd-Trimethylenemethane (TMM) type and analogs) as reactive dipole partners are popular methods that provide highly functionalized, saturated ring systems often with high stereocontrol and atom economy (Scheme 1). Discovered in the early 1980s, they have been extensively covered in the review literature [8-16]. 

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